Summary:
Add a command in ldb so that users can print out tombstones in SST files.
In order to test the code, change the interface of LDBCommandRunner::RunCommand() so that it doesn't return from the program, but return the status code.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5615

Test Plan: Add a new unit test

Differential Revision: D16550326

fbshipit-source-id: 88ddfe6984bdcbb3a528abdd115089df09eba52e